The present invention relates to a method for measurement of optical or electrical signal sequences in an optical or electrical transmission system wherein a multitude of consecutive signals are periodically sampled at a specific sampling time with one or more adjustable threshold values. In addition the present invention relates to an eye diagram monitor for generation of an eye diagram representation of a signal sequence with at least one threshold decision circuit, at least one storage device, and an analysis device.
In communication systems signals rsp. signal sequences are transmitted via optical or electrical transmission links. Accordingly transmission links are qualified as optical or electrical links. Both types of links have in common that on its way from transmitter to receiver the signal sequence is degraded by different effects, in particular attenuated and distorted, so that the signal at the receiver differs from the signal at the transmitter. In order to avoid errors in the transmission of signal sequences it is necessary to process the received signal in such a way that in the receiver the correct signal sequence is identified.
Since several years optical fibers have been increasingly utilized for signal transmission at high data rates over long distances. Besides attenuation, noise and other effects, dispersion limits the maximum data rate and maximum transmission distance of an optical link. With increasing data rates and transmission distances signals are stronger distorted, so that appropriate signal processing is required in order to recover the original signal sequence from the received signal and to avoid errors during transmission.
Compensation of distortion especially caused by dispersion can be achieved in the optical domain as well as in the electrical domain, e.g. by pre-distortion of the signal in the transmitter and post-processing of the signal in the receiver. At low data rates this can be done statically, e.g. by
cascading different types of optical fiber with different dispersion coefficients or by means of dispersion compensated fiber or by one-time adjustment of electrical or optical filters. At high data rates static compensation is not sufficient because dispersion effects can be time-variant. Especially temperature, pressure, and torsion changes rsp. vibrations can result in time-variant dispersion so that dynamic dispersion compensation is required.
Through dispersion effects, such as e.g. polarisation mode dispersion or chromatic dispersion, distortion of the signal occurs which is caused by overlapping signal components with different polarisation modes or wavelengths. The different signal components propagate with different velocities in the optical fiber so that the signals arrive with diffuse timing at the receiver. In order to separate different signals at the receiver which are superposed caused by dispersion effects dynamic equalization is required wherein optical or electronic filters are adjusted dynamically.
For dynamic filter adjustment a decision is required whether the adjustment yielded a signal quality improvement or degradation. This can be achieved by means of a comparison between target data and actual data which can be obtained e.g. by means of error correction at the receiver.
In the beginning it has been stated that the invention pertains to a method for measurement of optical or electrical signal sequences. Whether one refers to a signal sequence as being electrical or optical depends on where the signal sequence is looked at in the system. Usually in the transmitter a signal sequence is electrically generated which is then converted into an optical signal and fed into the optical link, i.e. optical fiber. For recovery of the distorted and attenuated signal sequence which has been transmitted the optical signal is converted into an electrical signal.
Afterwards, data recovery is usually done by means of a clock and data recovery device (clock and data recovery module, CDR) which samples the signal sequence at an “optimum” time and stores the sampling result in a digital memory cell until its next sampling point. As a result, a data signal virtually without time and amplitude noise is available. For the identification of the optimum sampling point two clock and data recovery devices can be operated at different sampling points wherein the better signal can be used for data recovery and the sampling point of the other clock and data recovery is varied to find the optimum sampling point. However, this technique is applicable only if the signal is of sufficient quality so that it is ensured that the chosen sampling point is near the optimum sampling point. If distortion of the received signal is too high an assessment of the signal quality is necessary beforehand.
According to the state of the art direct measurement of signal quality of optical transmission systems is often done by measuring the eye diagram. The eye diagram is a very good means to identify errors of certain components of a transmission system and to make a quality assessment of the performance of the system. The eye diagram is constructed by overlay of similar signal sequences on the screen of an oscilloscope.
According to the state of the art for fast signal measurement usually an analog sample-and-hold circuit is used which is followed by an analog-to-digital converter. With this measurement an eye diagram representation can be constructed similar to the oscilloscope. The sample-and-hold circuit is required because the analog-to-digital converter is in the order of magnitudes slower than the signal which is to be measured and requires a constant input signal. The sample-and-hold circuit consists of a storage element, usually a capacitor, and an electrical switch which is closed for an extremely short time interval so that the storage element is set to the value of the currently measured signal.
With increasing transmission rate the use of a sample-and-hold circuit becomes challenging with the time-window for the acquisition of the sampling value shortening and a maximum limit for the charge current the capacitance of the storage capacitor has to decrease. At data rates of up to 10 Gb/s and more, especially at a data rate of 40 Gb/s, the storage capacitor would require a capacitance which is lower than the parasitic capacitances of an integrated transistor, so that noise, leakage currents, and crosstalk would strongly degrade the accuracy of the measurement or make it impossible.
It is one object of the present invention to provide a method for measurement of optical or electrical signals of an optical transmission system by which signal quality can be measured even at very high data rates and an eye diagram representation of the signal sequence can be generated.
It is a further object of the present invention to provide an eye diagram monitor which can be realized at data rates of several Gb/s in a very simple manner, i.e. with few devices.